[functorch] Change the xfail/skip mechanism (pytorch/functorch#731)
Previously if you didn't specify a "variant_test_name" a skip like
"skip('add')" would default to ALL OpInfos whose base name is 'add'.
After this change, skip('add') defaults to the ('add', '') OpInfo.
The motivation for this change was that it was too confusing to add
xfails. 'add.bar' fails, so you might add `xfail('add')`, but it turns
out that 'add.foo' actually passes and now the xfail yells at you.
Test Plan:
- wait for tests
